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Full text of "Drying Kieffer pears and the use of the dried product"

Historic, archived document 

Do not assume content reflects current 
scientific l<nowledge, policies, or practices. 



t 




CIRCULAR No. 450 NOVEMBER 1937 

UNITED STATES DEPARTMENT OF AGRICULTURE 
WASHINGTON, D. C. 




DRYING KIEFFER PEARS AND tIiE tlSE OF THE 
DRIED PRODUCT 



By C. W. Culpepper, physiologist, and H. H. Moon, assistant nomologist, Division 
oj Fruit and Vegetable Crops and Diseases, Bureau of Plant Industry 



CONTENTS 



Page 

Introduction 1 

Purpose of the work 2 

Material and methods 2 

Peeling and preparation of the fruit for drying. 2 

Subdividing the fruit 4 

Ripened compared with unripened fruit 4 

Treating the fruit with sulphur dioxide 5 

Shrinkage of the material 6 

Composition of the Kieffer pear as related to 

the drying procedure and quality of the 

dried product 6 

Behavior of the dried product when stored in 

atmospheres of different humidity 7 

Moisture-absorbing capacity of the dried prod- ' 

uct 9 



Page 

Packing the dried product.. 12 

Rate of moisture absorption 13 

Effect of size of segments upon moisture ab- 
sorption 14 

Refreshing the material 15 

Effect of temperature upon the rate of absorp- 
tion of water in soaking 15 

Rate of absorption of water by segments of 

fruit of different sizes 18 

Effect of adding different quantities of water 

for refreshing the fruit 19 

Uses of dried Kieffer pears 21 

Discussion 21 

Summary 22 

Literature cited 23 



INTRODUCTION 

For some time work concerned with the possibilities of improving 
the quality of the Kieffer pear, so that it may be more completely 
utilized than has been the case in the past, has been in progress in 
this laboratory. Lutz, Culpepper, Moon, and Myers (13) have 
shown that the temperature at which the material is ripened has a 
very striking influence upon the quality of the fruit when canned as 
well as when used fresh. It has been found that when the fruit is 
harvested at the proper time and ripened at 60° F. its quality is 
much better than when ripened at any higher temperature. It is 
evident that many of the objectionable characteristics usually present 
in the fruit are overcome in this way. Likewise it has been shown 
by Moon and Culpepper (18) that if properly ripened fruit is used a 
fair quality of preserve may be made, provided that the proper pro- 
portions of fruit and sugar are used and that the mixture is boiled to 
the proper temperature at the finish. It was considered desirable to 
carry out similar tests dealing with the drying of the material. 

In beginning the tests, it seemed necessary to obtain first a 
thorough understanding of the factors that influence the rate of 
drying, and a rather comprehensive study of these has consequently 
been made. The results of the work done in this connection have 
been stated in another report (10). A considerable number of other 



» Italic numbers in parentheses refer to Literature Cited, p. 23. 



2 



CIRCULAR 450, U. S. DEPARTMENT OF AGRICULTURE 



tests dealing with the drying of the material and the use of the dried 
product have been made. 

There seem to be no records describing in detail attempts to dry 
the Eaeffer pear. The drying of Bartlett pears is described rather 
fully by Cruess and Christie (8). They are apparently not dried 
to any extent outside of California, and the principal method used 
is sun-drying. 

PURPOSE OF THE WORK 

A number of tests have been made to determine the effects of 
different methods of preparation upon the quahty of the dried prod- 
uct and the differences that may be expected in products made from 
ripened and unripened fruit. The properties of the dried material 
have been studied, and its behavior in storage in atmospheres of 
different humidities has been determined. Tests deahng with the 
refreshing of the fruit and the preparation of the material for the 
table have also been carried out. It is the purpose herein to report 
the results of these tests. 

MATERIAL AND METHODS 

The fruit for these tests was grown at the National AgTicultural 
Research Center, Beltsville, Md., during the season of 1934. An 
ample quantity of material was harvested and a large portion of it 
was ripened in a room held at a constant temperature of 15.5° C. 
(60° F.). A quantity of the unripened fruit was dried immediately, 
but a large amount was placed iu storage at 0° C. (32° F.) until the 
experunental tests could be carried out. The material was fairly 
typical of Kieffer pears, iu general. 

The material was dried ia a small steam-heated dryer of the stack 
type, modified to permit the maldng of tests under widely different 
conditions, a detailed description of which has been given in another 
report (10). In most of these tests the temperature of the drying air 
as it entered the dryer was approximately 50° to 55° C, and it had a 
relative humidity of 9.2 percent. When the dryer was loaded the air 
velocity varied somewhat on different trays, ranghig from a fraction 
of a mile per hour to 3 miles per hour. Its relative humidity when dis- 
charged was around 60 percent and the temperature had fallen to 35° C. 
Of course, there was not a very efficient use of the heat supplied, but 
the data are given to show that excellent drying conditions existed in 
the dryer. However, in many other tests smaU lots were dried under 
widely varying conditions. 

PEELING AND PREPARATION OF THE FRUIT FOR DRYING 

In considering the procedure in dryuig or dehydrating the fruit the 
question at once arises: Should the material be peeled? Or, of what 
significance is the difference in the quality of the peeled and impeeled 
product after drying? In considering the quality of the two products, 
it is well to bear in mind the losses to be expected from peeling and 
coring. To get some idea of the losses in peeling, 68 fruits, half of 
them ripened and half unripened, were weighed before and after peel- 
ing and before and after coring, and the difference was calculated as 
percentage loss. The fruit varied widely in size and shape. The 
peeling and coring was done by hand, using the customary guarded 
knife in peeling and the usual looped Imife for removing the cores. 



DRYING KIEFFER PEARS 



3 



In figure 1 the percentages of total losses and the losses in peeling 
alone have been plotted against the size of the fruit. It is readily 
observed that there was cousiderable variation in individual fruits of 
approximately the same size. In fruits weighing between 195 and 
200 g the extreme difference in total loss was from a little below 20 
percent to a little more than 30 percent. This was apparently due to 
differences in the smoothness of the surface and in the size and shape 
of the core, as well as to errors due to inabiUty to core the fruits in 
exactly the same manner. However, there is a distinct tendency for 
the total losses to be greater in the smaller fruits. This difference 
between the large and small fruits is not quite so marked as might have 
been expected from their great variance in size. It is evident that for 
fruit of average size (160 to 200 g) a loss of at least 25 percent may 



35 



30 



25 



15 



10 



























































































-Xs^ ° °° 

8 ooo o ° 






























■j 



80 100 



150 



200 250 300 350 

WEIGHT (GRAMS) 



400 



450 480 



Figure 1. — Relation of size of the Kiefler pear to the weight loss in peeling and coring: a, Total loss; b, loss 

after peeling only. 

be expected in peeling and coring. Except for very small fruits, the 
difference in size did not make a great difference in the percentage loss. 

In the losses from peeling alone, the differences in the large and 
smaU fruits are even less marked than the total losses. For fruits 
of average size a loss of about 13 percent may be expected. These 
results apply only to fruit peeled and cored with the regular guarded 
knife and coring spoon generally used in peeling the fruit in commer- 
cial practice. If an ordinary paring knife is used the losses may be 
very much greater than this. The fruit cannot be effectively or effi- 
ciently prepared by hand without the use of these instruments; if it 
has blemishes of any kind that must be removed the losses will be cor- 
respondingly greater. 

The losses were a little greater in peeling and coring the unripened 
fruit than the ripened. This was apparently due to the hard tissues 
of the unripened fruit that made it more difficult to cut to a uniform 



4 



CIRCULAR 450, U. S. DEPARTIHENT OF AGRICULTURE 



depth. A pear peeling, splitting, and coring machine has been in- 
vented (17) which greatly reduces hand labor, and the finished product 
is much more uniform than with hand peehng. 

In these tests the differences in quality of the finished product of 
the peeled and unpeeled fruit were so marked that it seems inadvisable 
to attempt to dry the material without peeling and coring. The un- 
peeled fruit was less attractive in appearance and less palatable when 
cooked because of the presence of the skins and stone cells. The time 
necessary properly to sulphur the material (p. 5) was very greatly 
increased and the rate of drying was much slower. The removal of 
the core is particularly important in determining quality, since the 
greater part of the stone cells are located in and near the core. 

SUBDIVIDING THE FRUIT 

The effect upon the rate of drying of slicing the fruit into segments of 
different size has been considered elsewhere ( 10 ). However, there are 
factors other than rate of drying to be considered. These tests 
indicate that the material may be dried satisfactorily when sliced in 
nearly any manner, under the proper conditions. As will be shown 
later, the refreshing of the material, like the rate of drying, is hastened 
by subdividing the fruit. However, these tests indicate that the 
very finely sliced material is not so convenient to prepare for the 
table or so attractive when prepared as fruit sliced into larger pieces. 
There is a greater tendency for the finely sliced material to be com- 
pletely disintegrated so that the original shape of the pieces is lost ; on 
the other hand, the dried halves are sometimes less conveniently 
prepared for the table than fruit sliced to four or eight segments. 
From these tests the writers have concluded that, all things being 
considered, division of fruit of average size into eight segments is 
about the best procedure. Segments of this size dry rapidly, are 
easily refreshed, are convenient to prepare for the table, and are 
attractive when served. 

RIPENED COMPARED WITH UNRIPENED FRUIT 

In order to study the characteristics of the ripened and unripened 
fruit, a large quantity of material was picked and divided into two 
portions, one of which was ripened at 15.5° C. (60° F.) prior to drying, 
and the other was dried mthout any ripening treatment. A portion 
of the unripened material was dried immediately after harvesting, but 
a quantity was put in storage at 0° C. (32° F.) until the ripened 
material was ready for use. 

Both lots were then peeled, cored, exposed to the fumes of sulphur 
for 4 hours (p. 5), and dried under identical conditions. The fruit 
that had been ripened, tested 3.1 pounds with the Magness pressure 
tester {15)] the unripened tested 13 pounds.^ There was a very 
marked difference in appearance between the ripened and unripened 
material. There was little or no browning in either case, as the sul- 
phuring appeared to be ample. The unripened material took on a 
grayish or white appearance during drying; the ripened was of a pale 
yellow or amber color. As drying proceeded, the differences in ap- 
pearance became more pronounced, so that the unripened appeared 
opaque with a whitish or grayish color and seemed starchy and coarse. 



DRYING KIEFFER PEARS 



5 



At the finish the ripened material appeared somewhat translucent and 
yellow or amber colored, and was somewhat suggestive in its appear- 
ance of certain candied fruits. When dried to 15-percent moisture the 
ripened fruit was soft and pliable, while the unripened was harder and 
less flexible. The unripened material held its shape more nearly 
perfectly than the ripened, the latter appearing to have undergone 
more shrinkage, but, as will be shown later, this was because of the 
manner of shrinking and not the volume. 

In order to compare the quality of the two lots of material after 
refreshing, a quantity of each, sliced into eight segments, was soaked 
overnight in enough water to make its moisture content about 85 
percent. The material did not absorb quite all the water. The 
following mxOrning the two lots were brought approximately to a boil 
and allowed to simmer slowly for 10 minutes with occasional stirring. 
The unripened material held its shape much better than the ripened, 
but was tough, woody, and lacked flavor; the ripened was tender, 
attractive in appearance, and seemed less gritty than the unripened. 
It was apparent that the unripened material was entirely unsatis- 
factory, while the ripened material was a fair product approaching 
the quality of the best dried Bartlett pears. 

TREATING THE FRUIT WITH SULPHUR DIOXIDE 

The pear, like most fruits, rapidly turns brown when the sliced 
material is exposed to the air. There are several methods of prevent- 
ing or minimizing this change, the most effective being the exposure 
of the material to the fumes of burning sulphur, or siilphur dioxide. 
This was the only method used in these tests. One lot of material 
consisting of peeled halves and fruit peeled and sliced into eight radial 
segments was dried without any treatment; another lot was exposed 
to the fumes of sulphur dioxide for 2 hours ; a third lot, consisting of 
peeled and unpeeled halves and fruit peeled and sliced into eight 
radial segments, was exposed to the sulphur dioxide for 4 hours; and 
a fourth lot, of peeled and unpeeled halves, was exposed to the fumes 
for 7}^ hours. These tests indicate that 2 hours' sulphuring is 
decidedly inadequate for fruit sliced into halves and scarcely sufficient 
for fruit sliced into eight segments; 4 hours' sulphuring seemed to be 
entirely sufficient for the peeled fruit, but was still inadequate for the 
unpeeled fruit. The unpeeled fruit sulphured for 7)^ hours was 
still insufficiently sulphured, and it was concluded that it would 
require 12 or more hours' exposure to attain the desired color. 

The difference in the color of the sulphured and unsulphured fruit 
was quite marked. The unsulphured fruit was quite brown in the 
case of the halves, but somewhat less so in the fruit sliced into eight 
segments. When both products were tasted shortly after drying, the 
flavor of the unsulphured material was much superior to that of the 
sulphured fruit. After refreshing and cooking, the difference is less 
marked, but the flavor is still decidedly more pleasing in the unsul- 
phured material. However, unless the storage conditions are very 
good the unsulphured product gradually loses much of its natural 
flavor, with the result that after 9 to 12 months under ordinary storage 
conditions the sulphured material is equal or superior in flavor to the 
unsulphured fruit. 



6 



CIECULAE 45'J. U. S. DEPaRTAIEXT OF AixRICULTrRE 



SHRINKAGE OF THE MATERIAL 

A visual mspection of the dried pieces of fruit revealed that the sur- 
face did not spht or crack appreciably during drying, which indicated 
that the fruit must shrink in all dimensions. In order to ascertaiu the 
amoimt of shrinkage., an eighth section was taken from each of four 
ripened fruits that had been peeled and cored. Sections were weighed, 
their volume was measured by immersing in ether, and the specific 
gravity calculated. The sections were then dried at 60^ C. in a current 
of au' having a velocity of 4.9 miles per hour and a relative humidity 
of 4.2 percent. The pieces were agaiu weighed and their volimie 
measured in the same way. The test was repeated six times and the 
results averaged. The fresh material at the beginoing had a specific 
gravity of 1.042 and after drying the specific gravity was 1.193. The 
Toss in weight was S4.9 percent of the fresh weight and the loss in 
volimie SS.l percent of the original volume. The percentage loss in 
volume was a httle more than the percentage loss in weight, which 
resulted in the increase in the specific gi'avity of the dried product. 
The volume of water lost was practicaUy ec^ual to the decrease ia the 
volmne of the fruit. 

It may be inferred that there is vety little air in the fresh fruit and 
very little in the dried material, so that httle if any is taken up or 
driven out of the material in drying. A quantity of ripened fruit that 
lost S4.9 g in weight lost S4.4S cc in volmne. A test was carried out 
upon the tmripened material in exactly the same way with practically 
the same result. Although the ripened fruit appeared to shrink more 
than the unripened. the actual shtinkage was the same in both cases. 
TVhen drying is done at a temperature of 60^ C. there is a tendency for 
the ripened material to flatten out somewhat as a result of the action of 
the heat and gravity. The tmripened pieces do not flatten out in this 
way. The apparent difference in the shrinkage is consequently due to 
distortion and flattening of the form of the soft ripened fruit, which 
iucreases as higher temperatures are used, so that it may become quite 
pronoimced at 70^. 

COMPOSITION OF THE KIEFFER PEAR AS RELATED TO THE 
DRYING PROCEDURE AND QUALITY OF THE DRIED PRODUCT 

^lany of the qualities of the Kiefi'er pear as weh as those of the prod- 
ucts prepared from it are attributable to or related to its chemical 
composition. The data presented in table 1 and taken from the report 
of Lutz, Culpepper, Moon, and Myers (13) show that the total sugar 
content of the Kiefler pear ranges between 6.63 and 7,79 percent with 
a titratable acidity ranging between 0.175 and 0.229 percent. In 
various analyses of the Bartlett pear reported by Magness {14), the 
sugar content of the frint when ripened after picking at the proper 
stage ranged between 8 and 10 percent, with titratable acidity ranging 
from 0.246 to 0.39 percent. 

Both the sugar and the acid content of the Banlett pear are con- 
sequently somewhat higher than those of the Kiefl^r. but the ratio 
of acid to sugar does not difl'er greatly in the two fruits. Judging 
from its composition as compared vith the Bartlett it appears that 
the Kieffer pear should yield a fair product when dried. 



DRYING KIEFFER PEARS 



7 



Table 1. — Firmness and composition of Kieffer pears at time of harvesting (Oct. 25) 
and after 20 days in storage at various temperatures expressed on the fresh-weight 
basis 



Temperature of storage 
(° F.) 


Pressure 
test 


Solids 
soluble 
in al- 
cohol 


Solids 
insolu- 
ble in 
alcohol 


Total 
solids 


Total 
sugar 


Titrat- 

able 
acidity 


Total 
astrin- 
gency 


Proto- 
pectin 


Ratio of 
soluble 
pectins 
to pro- 
topectin 




Pounds 


Percent 


Percent 


Percent 


Percent 


Percent 


Percent 


Percent 


Percent 


No storage 


12.0 


12. 92 


3. 23 


16. 15 


6. 90 


0.208 


0. 067 


0. 44 


0. 65 


40 


11. 1 


12. 60 


3. 38 


15. 98 


7. 05 


.229 


.096 


.41 


.77 


50 


6.5 


11.92 


3. 29 


15. 21 


6. 69 


.218 


.095 


.31 


1.33 


60 


4.0 


11. 56 


3. 23 


14. 79 


7. 74 


.205 


.075 


.20 


1.89 


70-— 


7. 1 


11. 14 


3. 50 


14. 64 


6. 63 


.180 


.087 


.27 


1. 15 


80 


10.6 


12. 26 


3. 50 


15. 76 


7. 15 


.202 


.095 


.43 


.73 


90 


11.2 


13. 04 


3. 72 


16. 76 


7. 25 


.225 


. 113 


.46 


.62 


100 


13.0 


15. 16 


3. 86 


19. 02 


7. 79 


.175 


. 106 


.47 


.78 



The dry matter of the Kieffer pear ranges between 14 and 17 percent, 
which compares favorably with that of the Bartlett pear and with that 
of many other fruits that are extensively dried. The greater part of 
the dry matter is sugar and other soluble materials, which should make 
the dried product easily refreshed and cooked in preparing it for table 
use. A somewhat greater proportion of the total sohds of the Kieffer 
pear consists of alcohol-insoluble materials, the difference from the 
Bartlett being due largely to the stone cells which consist chiefly of 
lignocellulose. Crist and Batjer (4) have studied extensively the 
stone cells in the Kieffer and other pears. The presence of the stone 
cells is one of the most objectionable featiu-es of the Kieffer pear from 
the standpoint of the dried product, for they become more apparent 
during the drying process, especially in the unripened material. The 
tannin content is as low, or lower, than in most of the fruits that are 
dried, which makes the difficulties due to discoloration less than in 
many fruits. 

The total pectin content of the Kieffer pear is not high, but there 
is a considerable increase in the soluble pectin during ripening, which 
probably accounts for much of the differences in the texture and 
quahty of the products made from the ripened and the unripened fruit. 

BEHAVIOR OF THE DRIED PRODUCT WHEN STORED IN 
ATMOSPHERES OF DIFFERENT HUMIDITY 

The extent to which a product is preserved by drying depends upon 
the deterioration that may occur in storage subsequent to drying, and 
its value or usefulness depends to a considerable extent upon the 
physical characteristics of the dried product. One factor in the 
storage conditions that may be of much importance is the humidity of 
the air. Gore and Mangels {11) have emphasized this factor in a 
study of the deterioration of raw-dried vegetables in storage. The 
influence of different humidities depends upon the physical properties 
of the material. A number of tests have been made to determine the 
physical characteristics of the material and its behavior in storage. 
The method employed was essentially that used by Culpepper and 
Caldwell (5) in studying the effect of atmospheric humidity on the 
rate of deterioration of stored evaporated apples. 

Material prepared in several different ways was stored in lots in 
atmospheres containing various percentages of moisture, and the 
behavior of the material was noted. Small storage chambers were 



8 



CIRCULAR 450, U. S. DEPARTMENT OF AGRICULTURE 



prepared by placing a graduated series of solutions of sulphuric acid 
of the proper concentration in large inverted bell jars which were 
covered with glass plates and made tight by applying stopcock grease 
or petrolatum to the contact surfaces, and the material was placed in 
the atmospheres above these sulphuric-acid solutions. Eleven 
different humidities ranging from approximately 100 percent to 
practically zero were thus provided. Fruit that had been peeled and 
sliced into eight segments was used. One lot was ripened and sul- 
phured ; another was ripened and unsulphured ; a third was unripened 
and sulphured; and a fourth was unripened and unsulphured. The 
material was placed in loosely woven muslin bags, and these were then 
put in the moisture chambers and held in storage at a temperature of 
20° to 23° C. for 6 months, when it became necessary to discontinue 
the test. The material was examined at frequent intervals to note 
its behavior as to color, flavor, and other characteristics. 

The results of the tests as to color and flavor are given in table 2. 
The numerical values are based upon the personal preferences of the 
judges, chiefly members of the laboratory where the work was done. 
A reading of 1 represents the most desirable product; a rating of 10 
represents the least desirable or the poorest. 

Table 2. — Changes in color and flavor of dried pears when stored at different humidi- 
tiesy the values given being numerical ratings based upon the personal preferences of 
the judges 



[1 represents the most desirable quality; while a rating of 10 represents the most undesirable, or poorest] 





Ripened sulphured 


Unripened sulphured 


Relative humidity 
(percent) 


1-month's storage 


6-months' storage 


] -month's storage 


6-months' storage 




Color 


Flavor 


Color 


Flavor 


Color 


Flavor 


Color 


Flavor 





1 


2 


1 


2 


3 


■5 


3 


5 


4.96 


1 


2 


1 


2 


3 


5 


3 


5 


12.89 


1 


2 


1 


2 


3 


5 


3 


5 


21.44 


1 


2 


1 


2 


3 


5 


3 


5 


33.30 


1 


2 


2 


3 


3 


5 


3 


6 


48.84 


1 


2 


3 


3 


3 


5 


4 


6 


62.28 


2 


3 


7 


4 


4 


5 


5 


6 


70.82 : 


3 


6 


8 


5 


6 


6 


8 


7 
9 


83.27 


4 


6 


9 


7 


8 


8 


9 


94.20 


6 


7 


10 


10 


9 


9 


10 


10 


100 


7 


7 


10 


10 


9 


9 


10 


10 














Ripened unsulphured 


Unripened unsulphured 


Relative humidity 
(percent) 


1-month 


s storage 


6-months' storage 


1-month's storage 


6-months' storage 




Color 


Flavor 


Color 


Flavor 


Color 


Flavor 


Color 


Flavor 





5 




5 


1 


6 


4 


6 


4 


4.96 


5 




5 


1 


6 


4 


6 


4 


12.89 


5 




5 


1 


6 


4 


6 


4 


21.44 


5 




5 


1 


6 


4 


6 


4 


33.30 


5 




6 


2 


6 


4 


7 


6 


48.84 


6 




6 


2 


7 


5 


7 


6 


62.28 


6 


2 


8 


4 


7 


7 


8 


8 


70.82 


7 


4 


9 


6 


7 


7 


9 


8 


83.27 


8 


5 


9 


9 


8 


8 


9 


9 


94.20 


8 


9 


10 


10 


8 


9 


10 


10 


100 


9 


9 


10 


10 


9 


9 


10 


10 















DRYING KIEFFER PEARS 



9 



It is evident that the dried product deteriorated rapidly in atmos- 
pheres of high humidity. At the end of 1 month in saturated air the 
material had become quite brown and greatly altered in flavor. Molds 
were beginning to develop in the unsidphured lots. The sulphured 
material was somewhat better preserved than the untreated, but both 
were quite brown. At the end of 6 months all were quite brown, so 
that there was not very much difference in the materials treated in 
different ways. Molds were more in evidence in the unsulphured lots. 
All lots had strong or abnormal flavor and were considered unusable as 
food. In a relative humidity of 83.27 percent both color and flavor 
were considerably better, at 62.28-percent moisture there was a 
decided improvement, and in the 3 3. 3 -percent relative humidity only 
a very small amount of change could be detected at the end of 6 
months. The material stored in air of 48.84-percent relative humidity 
retained its normal color and flavor remarkably well. No change in 
color or flavor was noted in the material in perfectly dry air, and it is 
concluded that the material would remain unchanged indefinitely. 
While the materials that had been treated and dried in different ways 
were quite different at the beginning of the storage they all deteri- 
orated in very much the same manner. Molds did not develop on the 
sulphured material even in the air saturated with moisture during the 
entire storage period. 

It was necessary to discontinue the storage at the end of 6 months, 
but it was concluded that the major part of the change that might be 
expected to develop had occurred during this time and that further 
change would be much less rapid and much less apparent than that 
which occurred during the first 6 months. 

MOISTURE-ABSORBING CAPACITY OF THE DRIED PRODUCT 

For the tests upon the moisture-absorbing capacity of the dried 
material, two lots of fruit — one ripened, the other unripened — were 
peeled, each fruit sliced into eight segments, and dried without sul- 
phuring or any other treatment at a temperature of 50° C. Following 
drying, the material was placed in a vacuum oven (at 28 pounds 
negative pressure) over calcium oxide and dried for 48 hours longer 
at 70°. From this material lots were weighed out, placed in loosely 
woven muslin bags in the moisture chambers already described (p. 8), 
and kept in storage at a temperature between 20° and 23° for 6 months, 
at which time equilibrium had practically been established. Several 
times during the period they were weighed and the moisture content 
of the material calculated. The water absorbed was also calculated 
as percentage of the original dry weight of the material. Table 3 
shows the relative humidities present in the various chambers and the 
concentration of the sulphuric acid solutions used to produce them; 
it also gives the results of the test in percentage of the water absorbed 
and the moisture content of the material. In the high humidities the 
result is given of the last weighing made before mold growth vitiated 
the result. The moisture content of each lot of material may be con- 
sidered as the equilibrium moisture or the moisture content at which 
the material has reached equilibrium with the air of the chamber in 
which it was stored. 



1255°— 37 2 



10 CIRCULAR 450. U. S. DEPARTMENT OF AGRICULTURE 



Table 3, — Percentages of water absorbed and the moisture content of peeled, un~ 
sulphured Kieffer pears dried with and without previous ripening when stored in 
atmospheres of different humidities 



Concentration of H3SO4 Cpercent) 


-Keiative 
humidity 
of storage 
chamber 


Unripened 


Eipened 


^'^ater 
Bbsorbed 


Moisture 


\» ater 
absorbed 


Moisture 




PCTC€7lt 


PtTC6Tlt 


Pncent 


PeTCSTlt 


Percent 


U — - - -- 


100 


65. 1 


" 39.4 


68. 1 


40.50 




94.2 


54.8 


35.4 


55. 5 


35. 70 


24.26 


83. 27 


33.2 


24. 94 


34.0 


25. 40 


33.10 


70. 82 


24.8 


19. 88 


24.9 


19.96 


37.69 


62. 28 


18. 1 


15. 30 


17.9 


15. 19 


43.75 


48. 84 


11. 1 


9. 89 


10.3 


9. 32 


52.13 


33. 30 


5. 30 


5.01 


3. 70 


3. 57 


57.65 


21.44 


2. 90 


2.8 


.46 


.46 


64.47 


12. 89 


1. 18 


1. 17 


.32 


.32 


73.13 


4. 90 










96.0 


.00 





















It is noted that in a saturated atmosphere the dried Kieffer pears 
absorb large quantities of water. The dry material of the ripened 
fruit absorbed 68 percent of its weight of water, or enough to make its 
moisture content 40.5 percent. With lower humidities the amount 
of water absorbed becomes less as the humidity of the atmosphere 
becomes less. 

It is noted that the difference in behavior in atmospheres of different 
humidities of the ripened and the unripened material is quite small 
and from a practical standpoint appears unimportant. 

This entire series of tests was not repeated, but the tests for humidi- 
ties of 94.2 and 100 percent were repeated several times with diJfferent 
lots of dried material. The quantities of water absorbed in the re- 
peated tests were frequently considerably higher than in the series for 
which data have been given. In one case the material absorbed ap- 
proximately 100 percent of water, or enough to make the moisture 
content 50 percent. This was thought to be due to differences in the 
original raw materials, but there was also some variation in the tem- 
perature of the room containing the storage chambers, which may have 
been partly responsible for the variation. There was a tendency at 
these high humidities for the material to weep during handling in 
weighing, and it was not always certain that some loss did not occur 
in this way. 

In examining the moist and the dry material it was apparent that 
swelling occurred as water was absorbed. No measurements of the 
swelling were made, but, judging from the shrinkage in drying, it would 
be expected that the amount of swelling would be almost exactly equal 
to the volume of water absorbed. 

The ripened material that had been stored in saturated air or in air 
having 94.2 percent relative humidity was quite soft and had very 
little tendency to regain its shape after being deformed by com- 
pression. It was very stick}^ and the pieces would adhere when 
pressed together and might be squeezed into a solid ball. The sirupy 
material would stick to the hands, but no liquid could be squeezed 
out. The unripened material behaved similarly, but was perhaps a 
little less sticky and a little less plastic, or had a little less tendencA" to 
remain deformed after compression. 



DRYING KIEFFER PEARS 



11 



The material stored in air of 83.27-percent relative humidity be- 
haved in somewhat the same way, appearing to be a little less soft and 
sticky. 

The lots in air of 70.82-percent relative humidity were distinctly 
less soft but were still soft with little elasticity, and the pieces would 
adhere when pressed together tightly with the hands. If shghtly 
compressed they would tend to separate to a certain extent, owing 
to the resiliency of the material. If an individual piece was bent it 
had a tendency to regain its former shape. The material was still 
quite sticky to the hands. 

There were distinct differences between the ripened and the un- 
ripened fruit after storage at a relative humidity of 70.82 percent. 
The unripened fruit was more elastic and less sticky, but the pieces 
would adhere when tightly squeezed between the fingers. It appeared 
much drier than the ripened fruit, although the moisture content was 
very nearly the same, being 19.88 percent for the unripened fruit 
and 19.96 percent for the ripened. 

In air of 62.28-percent humidity the ripened material showed con- 
siderable decrease in the tendency to stick together, and the pieces 
had a greater tendency to regain their former shape when bent. The 
pieces would stiU adhere if very tightly squeezed together with the 
hands. The unripened material also appeared less plastic and harder 
than in the higher humidities. 

In air of 48.84-percent humidity the ripened material was pliable 
but tended to fall apart when a handful of the pieces were tightly 
squeezed together. The pieces could still be made to adhere if they 
were pressed tightly together for several minutes. The pieces did not 
break when bent but tended to regain their former shape. 

The unripened material appeared hard and dry, but the pieces did 
not break when bent and tended to regain their former shape. It ap- 
peared distuictly drier than the ripened material; but, as in the above 
cases, this was not true, for the moisture content was nearly the same. 

The ripened material in air of 33.3-percent relative humidity was 
only shghtly pliable and would snap into two pieces when sharply 
bent. The pieces could not be made to stick together by pressing 
between the fingers, but were not dry enough to grind by hand in a 
mortar. 

The unripened material was much more brittle, was tougher, harder, 
and seemed drier than the ripened fruit. 

The ripened material in air of 21.44-percent relative humidity was 
hard and brittle. It could be ground coarsely in a mortar with a 
pestle but could scarcely be converted into a fine powder. 

The unripened material seemed a httle harder and drier than the 
ripened, but it could scarcely be made into a flour by grinding in a 
mortar with a pestle. 

The ripened material in air of 12.89-percent relative humidity was 
hard and brittle and could be converted into a powder, but there was 
some tendency to stickiness. The unripened material seemed a httle 
harder and more brittle but behaved in nearly the same way on 
grinding. 

In air having 4.9-percent relative humidity or less, the material 
could be ground to a flour. Although the appearance of the ripened 
and unripened materials was quite different, they behaved in very 
nearly the same way on grinding. 



12 CIRCULAR 45':'. U. S. DEPARmEXT OF AGRICULTURE 



From i':ie results of tliese tesrs it may be concluded that the moisture 
:o:i:r:it of the mate:if:-, ^-hich is m^s: desirable from the standpoint 

dai-dLing and pac>^:i_', is between 10 u::d 15 percent. The product 
with 10-percent moisture appears a httle too diy. and that with 15 
percent is a httle too sticky. The changes in appearance in the prod- 
uct with 15-percent moisture in the course of 6 months or a year are 
not very important, because the color ajid flavor remain very good. 
The material is phabie and is readily refreshed. However, it is judged 
that if the material with 15-perce:i t :::nisTure is to be stored for a period 
longer than 1 year it will deteri . : ; : ;ioeably in color and flavor 
unless air or oxygen is exclude:-. F^r _::"_g periods of storage with 
acce^ of air the moisture shou.;'. e ".\::ed below 10 percent. It is 
also evident that the materif l sli: be placed in a package which 
excludes moisture, for no i:i:;::er Ljw dry the material may be at 
the nnish of drying it will take up suflicient water from a moist at- 
mosphere to cause the product to spoil completely. 

PACKING THE DRIED PRODUCT 

Experience accuu^ulated in the course o: work with the storage of 
dried Kieuer pears and other dried fruits Ov:. Os to the be-irf tk:.t some 
practical suggestions in regard to their pucoAiig and preservation may 
be of considerable service to those inexperienced in the work. 

The upper hniit of moisture content that commercial evaporated 
apples may contain is fixed by Federal regiilation at 24 percent, 
that of prunes at 25 percent, and that of peaches and apricots at 26 
percent. The regulations do not prescribe an upper hmit of moisujre 
for evaporated pears,, but the similarity of the product to dried apples 
would indicate that 24 percent sho^uLd be taken as a limi t for dried 
pears as packed. Commercial packages f er dried fruit range from 
wooden boxes of 25- and 50-pO'jJid capacity t : 5-pound, 2-pound, and 
12-ounce paper cartons. The wooden b :o:-s are hned with two or 
more layers of waxed paper, so folded as to protect the contents against 
entrance of dirt and insects and to reduce contact with air and access 
of moisture: the paper cartons may or may not have an inner lining, 
but are usually protected by an outer covering of waxed paper or 
cellophane desismed to make them moisture and insect proof. 

Xo matter what t^e of package is used, the material is packed into 
it imder rather heavy pressure, which compresses the fmit into a 
compact block. This method of packing reqtures that the material 
have a moisture content sufficiently high to c;;:-e e-.L^s':::' o^d rub- 
"bery, so that it may be strongly compressed vatLout bre^imug c^r per- 
manently distorting the individual pieces. As a commercial method 
this has the advantage not only of reducing the bulk of the material 
and the cost of containers but the compression of the material reduces 
access of air and moistiure and consequently slows deterioration. It 
has the disadvantage that the material will take up water somewhat 
readily if exposed to a warm, moist atmosphere. This is prevented 
in the smaller cartons by sealing them into outer wrappings of 
moisture-pr' : f puper. but with the larger boxes that must be held for 
prolonged peric'os it is necessary to resort to cold storage. 

It would be advisable to dry material intended for home consump- 
tion to 10 percent or at most not more than 15-percent moisture 
content and to use tin cans having closely fitted shp tops or friction 
tops as storage containers. Heavy waxed paper or glassine bags are 



DRYING KIEFFEll PPLIRS 



13 



also satisfactory if they can be closed tightly enough to exclude 
insects and moisture. Un waxed paper or cloth bags are unsuitable, 
for the obvious reason that they do not exclude air and moisture and 
for the further reason that while such bags will exclude the Indian 
meal moth and other larger insect pests of dried fruit, some of the 
small beetles that sometimes attack dried fruit will cut through such 
paper or make their way through the meshes of the cloth. To be 
satisfactory, a container for dried fruit must be made of material 
that insects will not tunnel through, and it must be so made that it is 
practically airtight when closed. 

RATE OF MOISTURE ABSORPTION 

In order to obtain information on the rapidity of moisture absorp- 
tion, the lots of material employed in the tests just described were 
weighed at short intervals during the first 2 or 3 weeks of the storage. 
The results have been calculated as percentage of water present in 
the material and as percentage of water absorbed. The results for 
the ripened material are given in table 4, and in figure 2 the moisture 



z 30 




I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 2! 

TIME- ( days) 



Figure 2.— Moisture content of dried Kieffer pears after storing in atmospheres of different relative humid- 
ity for different lengths of time. The material was stored at 20° C. and without any mechanical circulation 
of the air. The material was filled loosely into coarsely woven muslin bags: a, 100-percent relative hu- 
midity; h, 83.27-percent relative, humidity; c, 62.28-percent relative humidity; d, 48.84-percent relative 
humidity. 

percentage has been plotted against the time stored in the humidity 
chambers. It will be noted that the material absorbed water rather 
rapidly for the first few days and that the rate of absorption gradually 
decreased as equilibrium was approached. The greater part of the 
water was absorbed during the first 10 days, and at the end of 20 days 
equilibrium was very nearly established. 

The differences in the quantities and rates of absorption in atmos- 
pheres of different humidity are very clearly shown in figure 3. Of 
course, the result of a test of this character will vary greatly with 
differences in temperature and degree of exposure of the fruit to the 
humid air. If the temperature had been higher the rate of moisture 
absorption would have been greater; if the fruit had been tightly 
packed instead of loosely placed in muslin bags the moisture absorption 
would have been slower. Likewise, if the air had been circulated 
rapidly the rate of absorption would have been increased. The size 
of the segments into which the fruit is sliced will also have an influence. 



24 CIRCULAR 450, U. S. DEPAliTMEXT OF AGRICULTURE 

The extent to which temperature, air velocity, and other methods of 
exposure affect the rate of absorption was not studied, but some tests 
were made with fruit sHced into different -sized segments. 



Table 4. — Rate of moisture absorption in initially moisture-free material in atmos- 
pheres of differe7it humidity expressed as percentages of water ahsorhed and moisture 
present in the material 



Relative humidity 
(percent) 


0.76 dr.y 


1.70 days 


2.76 days 


3.75 days 


ZMoisture 


Water 
absorbed 


]Moisttire 


Water 
absorbed 


^Moisture 


Water 
absorbed 


Moisture 


Water 
absorbed 


100 

S3.27 

62.28 

48.S4 


Perce rd 
8.90 
6.3 
3. 9S 
1.56 


Percent 
9. 77 
6. 7 
4. 1 
1. 58 


Percent 
15. 94 
10. OS 
6. 64 
2. 44 


Percent 
18.9 
11.2 
7. 1 
2.5 


Percent 
19. 57 
12. 71 
8.54 
3. 33 


Percent 
24.3 
14.5 
9.3 
3.44 


Percent 
24.90 
15.85 
9. 80 
4.00 


Percent 
31.6 
17.4 
10.8 
4. 16 


Relative hmnidity 
(percent) 


4.75 days 


6.75 days 


10.75 days 


20.82 days 


Moisture 


Water 
absorbed 


Moisture 


Water 
absorbed 


Moisture 


Water 
absorbed 


Moisture 


Water 
absorbed 


100 

83.27 

62.28 

48.84 


Percent 
26.90 
17.24 
10.68 
4.08 


Percent 
36.8 
20.8 
11.9 
4.25 


Percent 
31.0 
19.80 
12. 13 
4. 82 


Percent 
44.9 
24. 6 
13.8 
5. 06 


Percent 
31.90 
21.71 
13.87 
6. 11 


Percent 
46.8 
27. 7 
16'. 1 
6. 50 


Percent 
39.4 
24.94 
15. 30 
8. 53 


Percent 
48.5 
33.2 
18.0 
9.33 



EFFECT OF SIZE OF SEGMENTS UPON MOISTURE ABSORPTION 

The fruit for these tests was dried and stored at 100-percent relative 
humidity in the same manner as already described. The material 
was weighed at intervals during the storage period, and the results 
were calculated as percentage of water absorbed and percentage of 
moisture present in the material. The results are given in table 5 
and illustrated in figure 3 where the percentage of moisture is plotted 
against the time of storage. 



50 



uj 30 



























































i 






















/ 












































d. 


































/ 


















H 














































■ 















I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 

TIME (DAYS) 



Figure 3.— Moisture content of dried Kieffer pears sliced into segments of different sizes and stored in 
an atmosphere of 100-percent relative humidity for diflerent lengths of time. The air in the humidity 
chamber was not mechanically circulated, and the temperature of the storage was 20^ C: a. Fruit sliced 
mto 32 segments; 6, fruit sliced into 16 segments; r, fruit sliced into S segments; d, fruit sliced into 4 sea- 
ments; e, fruit sliced into halves. 



DRYING KIEFFER PEAKS 



15 



Table 5, — Water absorbed by and icater -present in dried pears sliced into segments 
of different sizes after storage in an atmosphere of 100-percent relative humidity 
for various periods. The material was moisture-free at the beginning of the test 



Storage period 
(days) 


Halves 


Quarters 


Eighths 


Si.xteenths 


Thirty-seconds 


Mois- 
ture 


Water 

ab- 
sorbed 


Mois- 
ture 


Water 

ab- 
sorbed 


Mois- 
ture 


Water 

ab- 
sorbed 


Mois- 
ture 


Water 

ab- 
sorbed 


Mois- 
ture 


Water 

ab- 
sorbed 


0.9 

2.48 

4.5 

7.2 

13.0 

20.0 


Percent 
5. 19 
14. 75 
20. 1 
24. 70 
29. 16 
31.6 


Percent 
5. 47 
17. 30 
25. 15 
32. 80 
41. 16 
46. 20 


Percent 
8. 86 
21.72 
26. 75 
30.65 
34. 23 
36. 1 


Percent 
9. 72 
27. 75 
36.51 
44. 19 
52.04 
56. 49 


Percent 
13. 18 
25. 21 
30.9 
34. 74 
38. 37 
39.4 


Percent 
15. 18 
33. 70 
44.71 
53. 23 
62. 25 
65. 10 


Percent 
18.00 
30.41 
36.0 
39. 5 
42.2 
42.3 


Percent 
21.95 
43. 69 
56. 25 
65. 28 
73. 01 
73.31 


Percent 
24.8 
41.8 
45.6 
45.8 
45.9 
45.8 


Percent 
32. 97 
71. 82 
83. 82 
84. 50 
84.84 
84. 50 



It is evident that the rate of absorption of moisture is tremendously 
increased in the fruit shced into small-size segments before drying. 
At the end of 2 K days the fruit cut into thirty -seconds had a moisture 
content of 41.8 percent, whereas the halves had only 14.75 percent. 
The thirty -seconds had almost reached equilibrium at the end of 4.5 
days, while the halves required approximately 35 days. From this 
it is evident that moisture penetrated the material comparatively 
slowly. It is also evident that the absorption of moisture from the 
air can be greatly lessened by storing the material in large masses 
with the pieces pressed tightly together, so that as little circulation of 
air as possible occurs between the pieces. This reduces the surface 
exposed to the air and minimizes the absorption of moisture. Of 
course, these results are comparative only, for, as already stated, the 
rate \viU vary with many other conditions. 

REFRESHING THE MATERIAL 

No matter how quickly a product may be dried or how satisfactory^ 
it may be in appearance, table quality, and other respects, its useful- 
ness w]R be greatly limited if it cannot be conveniently prepared for 
the table. The first step necessary in this preparation is the addition 
of the proper amount of w^ater. To add this w^ater, it is customary to 
soak the material for a certain length of time in water of a certain 
temperature. 

EFFECT OF TEMPERATURE UPON THE RATE OF ABSORPTION OF 

WATER IN SOAKING 

For these tests, ripened fruits weighing 180 to 190 g were peeled, 
cored, and sliced into eight segments, then dried at 60° C. until very 
dry; the remainder of the water w^as removed by drying in a vacuum 
oven at 70°. This assured uniformity as to moisture content, made 
the w^eighings directly comparable, and simplified the calculations. 

The material was soaked in large-mouthed Thermos bottles provided 
with stoppers through which thermometers extended. Baskets of 
such size that they could be rapidly and conveniently introduced or 
removed from the Thermos bottles for weighmg were made from thin 
sheet aluminum, the walls being perforated to allow free circulation of 
liquid. One of these baskets held the material during the soaking. 
The Thermos bottles kept the temperature fairly close to that desired, 
but when variations of 2° occurred an adjustment was made by the 
addition of a little hot or cold liquid. 



15 CIRCULAR 450, U. S. DEPARTMENT OF AGRICULTURE 



In order to submerge the material and to keep the individual pieces 
separated sufficiently to permit maximum absorption, it was necessary 
to use a rather large volume of water in proportion to the quantity of 
fruit. It was soon found that under these conditions so much sugar, 
acids, and other substances diffused out of the material as to leave some 
doubt whether the results represented accurately the amount of water 
absorbed. To partially correct this the soaking was done in an extract 
of the fruit itself. 

To prepare this extract a quantity of dried fruit was soaked for 24 
hours in enough water to make its moisture content 80 percent; the 
liquid was then squeezed out of the material and strained through 
muslin. This liquid contained approximately the concentration of 
sugars and acids that would be in the fruit after soaking if none 
diffused out. This method appeared very largely to correct for the 
diffusion of the solids out of the material during soaking in water, and 
it is believed that this concentration of solids in the water did not 
greatly affect the rate of water absorption. 

The procedure may therefore be described as follows: The liquid 
prepared as described was brought approximately to the desired 
temperature and introduced into the Thermos bottle. The aluminum 
basket was then introduced and the temperature adjusted to exactly 
the desired point. The basket was removed from the liquid, drained 
30 seconds, and weighed. The weighed material was placed in the 
basket, the pieces being kept as free from contact with one another as 
possible. The basket containing the fruit was then placed in the 
Thermos bottle, care being taken to see that all the fruit was sub- 
merged in the liquid. At the end of 5 or 10 minutes the basket was 
removed, allowed to drain 30 seconds, and weighed as quickly as 
possible. It was immediately returned to the Thermos bottle so that 
the fruit was out of the liquid not more than 1 to 1}^ minutes. Sub- 
tracting the original weight of dry fruit and the basket from the total 
weight gives the weight of the water absorbed. 

The material was weighed at approximate intervals until the end of 
24 hours. After 4 hours the rate of absorption was much slower and 
the results are not recorded (table 6). The results were calculated in 
terms of moisture content of the fruit. The tests were repeated two 
or three times and the results averaged. The temperature of the 
soaking liquid varied by approximately 10° intervals from 0° to 93° C. 
The radial segments of one-eighth of a fruit were selected for uniform- 
ity and weighed approximately 2.3 g each. The results are given in 
table 6 and illustrated in figure 4. 

It is noted that the moisture content increases very rapidly when 
the material is first put into the liquid. This increase is partially due 
to the layer of water held on the surface of the material, but even after 
making allowances for this the rate of absorption was very rapid. At 
53° C. the moisture content of the fruit had risen from 0.0 to 39.4 per- 
cent at the end of 10 minutes; at the end of 55 minutes it was 56.3 
percent. It appears that in soaking, water penetrates into the ma- 
terial more readily and rapidly than it diffuses out to the surface in 
drying. In drying, the osmotic forces and the forces of imbibition act 
to retard the passage of water to the surface, whereas in soaking they 
operate to hasten the penetration of water. Many other factors may 
also be concerned. 



DRYING KIEFFER PEAKS 




12 24 36 48 60 120 180 240 

TIME (MINUTES ) 



FiorRE 4.— Effect of temperature upon the rate of absorption of water by Kieffer pears, peeled, cored, 
and sliced into eight radial segments and dried to moisture-free condition: o, 93° C; 6, 73°- c, 53°,- d, 33°; 
e, 13°; /, 0°. 



Table 6. — Moisture content of dried Kieffer pears after soaking for different lengths 
of time at various temperatures. The fruit had been peeled and sliced, into 8 
radial segments and dried to moisture-free condition 



Tempera- 
ture of 
water 
during 
soaking 
(° C.) 



^loisture content of the fruit after- 



0_ 
13 
23 
33 
43 



10 
min- 
utes 



25 
min- 
utes 



Pd. 
25.4 
30. 43 
33.6 
35.9 
37.8 



Pet. 
36.9 
42.0 
44.0 
45.7 
47.4 



55 
min- 
utes 



Pd. 
44.6 
49.0 
50.9 
52.8 
54.6 



115 
min- 
utes 



Pd. 
53.5 
56.3 
57.9 
59.4 
60.9 



175 
min- 
utes 



Pd. 
58.0 
60.7 
62.1 
63.2 



240 j 
min- I 
utes il 



Pd. 
61.5 
63.2 
64.2 
65.5 



Tempera- 
ture of 
water 
during 
soaking 
(° C.) 



53 
63 
73 
83 
93 



Moisture content of the fruit after- 





25 


55 


115 


175 


240 


mm- 


min- 


min- 


min- 


min- 


min- 


utes 


utes 


utes 


utes 


utes 


utes 


Pd. 


Pd. 


Pd. 


Pd. 


Pd. 


Pd. 


39.4 


49. 1 


56.3 


62.2 


64.9 


66.7 


40.8 


50.6 


57.8 


63.5 






42. 1 


52.2 


59.7 


64.7 


66.2 


67.5 


43. 1 


53.7 


60.6 


65.9 






44.1 


55.3 


62. 1 


67.0 


67.9 


68.4 



The difference in the rate of absorption of water at high and at 
low temperatures is not so great as might have been expected when the 
difference in drying rates at high and at low temperatures is considered. 
At the end of 25 mmutes the moisture content was 36.9 percent at 
0° C, and 55.3 percent at 93°, while at the end of 4 hours it was 61.5 
percent at 0° and 68.4 percent at 93°. It is evident that the rate of 
absorption of water is of a ver}^ different order from the loss of water 
in drying in that heat or energy must be supplied, in order to evaporate 
the water, whereas energy is not required in water absorption. On 
the contrary, considerable amounts of energy are liberated in the 
absorption of water by the material. 

At the end of 24 hours the material had about reached equilibrium, 
so that no more water was absorbed. The moisture content of the 
material at this time ranged from 72 to 75 percent, hence was always 
a little lower than the moisture content of the fresh fruit before drying. 
Tests were also made upon the rate of absorption of water by the 



Ig CIRCULAR 450. U. S. DEPART^IENT OF AGRICrLTURE 



imripenecl material. The results indicate that the differences in the 
rate of absorption were so small as to be imimportant from a practical 
standpoint. Tlie mnipened material appeared to absorb water a 
little faster than the ripened, and the total amoimt absorbed was a 
Little greater. The moisture content of the material at the end was 
73 to 75 percent. 

As had been noted in the these studies, the material swelled or 
increased in volimie during the absorption of water. Dming the 
soaking the material did not quite regain its original volume before 
drying. It may be concluded from the shrinkage in drying that the 
swelling which occurs during soaking is almost exactly equal to the 
volume of the water absorbed. 

RATE OF ABSORPTION OF WATER BY SEGMENTS OF FRUIT OF 

DIFFERENT SIZES 

The material for these tests was prepared in the same manner as 
that for the tests on the effect of temperature, and the procedure was 
the same. Lots of fruit were subdivided into 2. 4, 8, 16, and 32 seg- 
ments. The fruit was uniform and an effort was made to slice the 
fruit symmetrically. After drying, the segments were further selected 
for uniformity. Xo record was kept of the weights of the individual 
pieces, but the whole fruits weighed 175 to 185 g. The soaking was 
carried out at a temperature of 23^ C. which remained practically 
constant throughout the tests. Each test was repeated three times 
and the results were averaged. The data are presented in table 7. 



Table 7. — Absorption of water by dried pears as affected by size of segment^ showing 
percentage of moisture absorbed after soaking for various lengths of time at :23° C. 



Size of piec« 


■u:e5 


or ^^->2- 


0-5 min- 
utes 


11.' min- 
utes 


175 min- 
ntes 


240 min- 
ures 


4S0 Ein- 
ures 


Quarters... . . 

Eishths 

Thirty-seconds 


Percent 
17. 2 
23. 5 
33.6 
40. 6 
5S. 1 


Pfrc:-:: 

29^5 
44.0 

5L0 

65. 6 


aio 

50.9 

58.2 

69. 2 


Perce Tit 
36. 1 
46.2 
57.9 
6o.O 


Percent 
39. 5 
52.0 
62. 1 
6S . 3 

-o ^ 


Perce- f 
43.5 
CO. 4 
64. 2 
69. S 
74. 


Percent 
53. 3 
63.2 
69.0 
72. 1 
74.0 



It may be noted that there is a very great diiference in the rate of 
absorption of water by segments of different sizes. At the end of 
10 minutes halves had absorbed 17.2 percent of water, while the 
material that had been sliced into 32 segments acquired a moisture 
content of 58.1 percent. At the end of 1 hour and 55 minutes the 
one thirty-second segments had a moisture content of 71.7 percent, 
whereas the halves had only 36.1 percent. 

The differences in rate and amoimt of absorption by pieces of 
different sizes for the first S hours of the soaking period are clearly 
shown in figure 5. The curves slope very sharply upward at first, 
gradually becoming horizontal toward the end of the soakiag period. 
The effect of the size of the segment on the rate of absorption of water 
is very similar to that of the size of segment upon rate of loss of water 
m drying (10^. In moisture-absorption tests the water must pene- 
trate into the material; in the drying tests the water must diffuse out 
to the surface. However, it may again be noted that the time re- 



DRYING KIEFFER PEARS 



19 



quired to dry the material is much greater than the time necessary 
for it to absorb water to its capacity. As is well known to be the case 
with other materials, the time necessary to reach equilibrium when 
the material is submerged in water is much less than that required to 
reach equilibrium in the case of absorption of water from a humid 
atmosphere. 




3 4 5 

TIME (HOURS) 

FiGUiiE 5.— Percentage of moisture content of dried Kieffer pears after soaking for different lengths of 
time: a, Halves; b, quarters; c, eighths; d, sixteenths; e, thirty-seconds. 

It is obvious that the refreshing of the material is easily accom- 
plished. With material sliced into eight or more segments it can be 
done almost as quickly as the fresh fruit can be peeled and cooked. 
Judging from the behavior of the material in these tests, the fruit 
sliced into eight segments can be more conveniently and attractively 
prepared for the table than that cut into segments of any other size. 

EFFECT OF ADDING DIFFERENT QUANTITIES OF WATER FOR 
REFRESHING THE FRUIT 

In refreshing any dried fruit it is advantageous to know the proper 
amount of water to add in order to give the product the characteristics 
most desirable for the particular form in which it is to be served on 
the table. It seemed reasonable to expect that an optimum moisture 
content would exist for the table product made of the dried Kieffer 
pear. A few tests were made to determine how the material behaved 
when different quantities of water were added. 

For use in these tests the fruit was peeled, cored, sulphured, sHced 
into eight segments, dried at 60° C. until most of the water had been 
removed, and then completely dried in a vacuum oven over calcium 
oxide at 70°. For each test 150 g of the dried material was placed in a 
round-bottomed flask and enough water added to make the moisture 



20 



CIRCULAR 450. U. S. DEPART^IEXT OF AGRICULTURE 



content the deshecl percentage. A reflux condenser was attacked to 
the flask, which was then phiced on a boilmg water bath and heated 
for 4 houi's. The flasks and contents were cooled overnight. Notes 
were then taken as to the ciuantity of water absorbed and the ap- 
pearance of the materiah The reflux condensers were removed and 
the contents of the flasks emptied into dishes and thoroughly stnTed. 
An efl'ort was made to determuie the consistency of the intact pieces, 
but no very satisfactory method was devised: however, this suggested 
that by maceratmg the material as finely as possible m an ordinary 
food chopper the tests might be satisfactorily made, Any liquid 
not absorbed by the material was added and thorouglfly mixed -^vith 
the groimd material. A consistency test was then made tipon this 
material, usmg a consistometer originally devised for measuring the 
consistency of greases and petrolatum m accordance with the specifi- 
cations of the American Societv for Testing Materials for test 
D-217-27T (18). 

In order to make satisfactory- comparisons of the consistency of 
materials -^dth Aridely A'aiying moisture percentages it was necessary 
to construct a special cone suitable for obtaining readings upon sub- 
stances of veiy thm consistency. A hollow cone that was found 
reasonably satisfactoiy for tliis purpose had waUs made of thin sheet 
aluminum with one-half inch of the point or vertex made of steel. 
The outer face of the cone had a uniform slope from the vertex to 
the base, and made an angle of approximately 15° with the axis of 
the cone. Its altitude was approxunately 8 cm. It was provided 
with an alimiinum attachment or stem for holding the cone in position 
for its release. The cone and supporting stem had a total weight of 
20 g and sank to a depth of approximately 64 nun in pure water. Tliis 
cone proved to be well adapted for measuiing the consistency of the 
products here under consideration. The tests were made at 20° C. 
In making the test, the material was placed in a suitable vessel, uni- 
formly mixed, and placed upon the instrument. The cone was then 
lowered imtil the tip was just in contact ^dth the sm^ace of the ma- 
terial, and the pointer of the recording deduce was adjusted to zero. 
The cone was then released, allowed to fall for 5 seconds measured by 
a stop watch, and the distance read from the scale and recorded. Three 
to five readings were made on each sample and the results averasred 
(table S\ 



Table S. — Corisisttncy of <:rkd K'.-:ftr pears after adding suficient water to maJce 
the moisture conien: la'-io'..-^ pt''iy: 'I'.ages as meas'urtd. by the distance to which the 
-material is penetrated by a tali cone weighing 20 g: temperamre of 'material, 20^ C. 



Moiiture contenT 
(percent ■ 


Depth of I 
penetra- 
tion 


[Moisture content 
rpereent) 


Depth of 
penetra- 
tion ' 


Moisture content 
(percent- 


Depth of 
penetra- 
tion 


50 


Mm 

7.9 
10.5 
15.1 


70 


Mm j 
18.8 
27.5 i 
32.3 


85 


Mm 
47.0 
58.1 


60 


75 


90 


65 


SO 









It is noted that the cone penetrated 7.9 mm in the material with 
oO-percent moisture and oS.l mm in the material ^dth 90-percent 
moisture. It is noted that as the percentage of moisture changes from 
80 to 90 there is a decided increase in the chstance to which the cone 



DRYING KIEFFER PEARS 



21 



penetrates, indicating a very great change in the consistency^ The 
increase in depth of penetration between 50- and 60-percent moisture 
is very much less. The results conform very well with the judgments 
as to the consistency formed after stirring the material or preparing 
it for table use. 

The material to which sufficient water had been added to make the 
moisture content 50 percent absorbed all the added water; and, after 
grinding the product had a consistency equal to or greater than that 
of good apple butter. With 60-percent moisture, the product was 
as thick as average apple butter. In the material with sufficient 
water added to make the moisture content 75 percent practically all 
the water was absorbed in the heating and soaking and the ground 
product had a consistency equal to or heavier than that of apple sauce. 
The material with enough water added to make the moisture content 
90 percent had a considerable quantity of unabsorbed. water and w^as 
very fluid after grinding. 

From these tests it is concluded that to have the most desirable 
consistency for use on the table the product should have a moisture 
content of approximately 75 percent. It is clear that a product of 
any desired consistency may be had by adding the proper amount 
of water. 

USES OF DRIED KIEFFER PEARS 

When the fruit of the Kieffer pear is ripened at the proper tem- 
perature, peeled, cored, properly sulphured, and dried under good 
drying conditions to a moisture content of 15 percent, the product 
obtained is attractive in appearance and phable to the touch, sw^eet 
and palatable to the taste, and comparable in quahty to the best dried 
Bartlett pears. The objectionable quahties are the presence of stone 
ceUs and a somewhat low sugar content, neither of which may be very 
serious. A product thus obtained may be prepared for the table in 
practically the same way as dried apples, peaches, raisins, and prunes. 

No attempt has been made to work out any new methods of pre- 
paring the material for the table or of combining it with other mate- 
rials to obtain appetizing dishes, although there are obviously various 
possibihties in these directions that would repay some attention. 
However, tests have been made upon the cooking quality of the dried 
material when prepared in most of the ways used in preparing evapo- 
rated apples, peaches, and prunes for the table. When stewed like 
peaches or prunes it appears advantageous to add sufficient sugar to 
make the concentration in the product as served about 20 to 30 per- 
cent. The product tends to darken somewhat in cooldng, but is 
attractive in appearance and pleasing in flavor. Other tests have 
shown that the dried product can be made into satisfactory pies by 
methods similar to those used for making pies of dried peaches. 

In these tests the product, if dried without previous ripening when 
cooked, was lacking in flavor, hard and woody in texture, and not 
attractive in appearance as compared with the ripened material. 

The unpeeled product even in the case of ripened material appeared 
quite inferior to the peeled material. 

DISCUSSION 

The studies of Lutz, Culpepper, and Moon {12) have shown that the 
Kieffer pear is much improved in palatability when ripened at 60° F., 
and those of Moon and Culpepper (18) and Lutz, Culpepper, Moon, 



22 CIRCULAR 450, U. S. DEPARTIMENT OF AGRICULTURE 

and Myers (13) have shown that the quahty of various products made 
from fruit so ripened is decidedly superior to that made from unripened 
fruit or from that ripened at either higher or lower temperatures. 
However, the fruit does not remain in good condition for a very long 
period after it has been ripened; changes continue to occur, and in a 
short time it begins to turn brown and loses distinctive flavor as a 
result of internal break-down. The fruit is much more susceptible to 
the attacks of fungi after ripening, so that once ripened it will soon 
perish unless it is preserved by some method. The hfe of the ripened 
fruit may be considerably extended by holding it at 32° F., but even 
with low-temperature storage the fruit cannot be kept in good edible 
condition for any extended period. Hence the usefulness of the fruit 
as a fresh product must be correspondingly limited. Where any con- 
siderable quantity of the fruit is produced some method of preserva- 
tion must be employed. The possibilities for canning the fruit or of 
making preserves from it have been discussed elsewhere (13, 18). 
There may be many instances in which it is more convenient and more 
economical to dehydrate the material than to preserve it in other ways. 

Because the fruit is grown over a very large portion of the eastern 
United States and therefore largely under rather humid conditions, it 
appears unlikely that sun-drying could be practiced to any extent in 
most of the area; also the fruit matures late in the season w^hen the 
days are shorter and the temperature lower, which still further mili- 
tates against successful employraent of sun-drying. It seems, there- 
fore, that drying by means of artificial heat is the only method which 
can be generally used for drying the fruit. Numerous writers have 
described drying equipment suitable for every condition and upon 
any scale from the simplest home-made cookstove or family driers to 
the large commercial dehydrators. This equipment cannot be de- 
tailed here, but a number of publications describing such equipment 
are included in the literature {1, 2, 3, 6, 7, 8, 16, 19, 20, 23), together 
with several papers of a more technical character dealing with the 
fundamental principles concerned in drying (5, 21, 22). 

SUMMARY 

Kieffer pears ripened at 60° F. for a sufficient length of time to give 
a pressure test of 2.5 to 3.5 pounds with the Magness pressure tester 
made a dried product much superior to that made from unripened 
fruit. When properly dried to a moisture content of 10 to 15 percent 
the ripened product was pliable, pale yellow in color, somewhat 
translucent, and when cooked made a table product that was mild and 
pleasing in flavor. The umipened product was hard, less flexible, 
opaque gray or white in appearance, and when cooked was tough and 
lacking in flavor. 

Drying the material without peehng gave a product inferior in 
texture and flavor to that of the peeled fruit. 

Fruit dried without sulphuring was brown and less attractive in 
appearance than that properly sulphured, but was superior in flavor 
when sampled after several months' storage unless the storage con- 
ditions had been poor. To sulphur properly the fruit sliced into halves 
required about 4 hours' exposure to the fumes of sulphur dioxide; and 
for fruit sHced into eight segments, about 2 to 3 hours. It required 
several times as long to sulphur the unpeeled fruit as that which had 
been peeled and cored. 



DRYING KIEPFER PEARS 



23 



The fruit may be dried when shced in nearly any manner if the 
proper drying conditions are provided. Because of the increased rate 
of drying, the ease of sulphuring, and the convenience of preparing it 
for the table, slicing the fruit into eight radial segments appears to 
be advantageous. 

The dried product of the Kieffer pear readily absorbs moisture from 
the air. In an approximately saturated atmosphere the dried product 
will absorb sufficient water to makes its moisture content 40 to 50 
percent. The equilibrium moisture of the material has been de- 
termined at 20° C. with various different humidities. 

Regardless of the treatment received, the dried product deteriorates 
rapidly in moist air. The rate and the total amount of change vary 
directly with the relative humidity. For the 6-months period of 
storage the previously ripened material that had been sulphured prior 
to drying and placed in an atmosphere of 48.84-percent relative 
humidity was in fairly good condition, having a moisture content of 
11.1 percent. It was concluded that if an extended storage period 
was necessary it would be advantageous to reduce the moisture con- 
tent of the material below 10 percent. The material rapidly deterio- 
rates in atmospheres above 70-percent relative humidity. 

The dried product of the Kieffer pear is readily refreshed by soaking 
in water. The material absorbs sufficient water to make the moisture 
content 70 to 80 percent, or not quite so much as was originally present 
in the fresh fruit. The rate of absorption increases with increase in 
temperature, but not to the extent that might be expected from the 
difference in the rate of drying at the different temperatures. 

The rate at which water is absorbed is very markedly increased by 
sHciug the fruit into thin segments before drying. The results have 
been plotted and curves drawn showing the relationship of size of 
segment to the rate of water absorption. 

The addition of sufficient water to make the moisture content 50 to 
60 percent gives a product which, when macerated, has a consistency 
about like that of apple butter. It has been concluded that about 75 
to 80 percent of water is the proper amount to add to the material to 
give it the proper consistency for use as a sauce. The addition of 
sufficient water to make the moisture content 85 to 90 percent gives a 
product that is entirely too thin for use in most of the ways in which 
it might be prepared for the table. 

The season at which the fruit matures is such that, in most of the 
area where the Kieffer pear is grown, it cannot be advantageously 
dried with solar heat alone. Drying equipment employing artificial 
heat will be necessary, except possibly in areas normally having very- 
hot, dry weather during the late autumn months. 

LITERATURE CITED 

(1) Caldwell, J. S. 

1919. FARM AND HOME DRYING OF FRUITS AND VEGETABLES. U. S. 

Dept. Agr. Farmers' Bull. 984, 61 pp., illus. (Revised.) 

(2) 

1923. EVAPORATION OF FRUITS. U. S. Dept. Agr. Bull. 1141, 64 pp., 
illus. 

(3) Christie, A. W. 

1926. THE DEHYDRATION OF PRUNES. Calif. Agr. Expt. Sta. Bull. 404, 
47 pp., illus. 

(4) Crist, J. W., and Batjer, L. P. 

1931. the STONE CELLS OF PEAR FRUITS, ESPECIALLY THE KIEFFER PEAR. 

Mich. Agr. Expt. Sta. Tech. Bull. 113, 55 pp., illus. 



24 CIRCULAR 450, U. S. DEPARTMENT OF AGRICULTURE 

(5) Cruess, W. V. 

1919. EVAPORATORS FOR PRUNE DRYING. Calif. Agr. Expt. Sta Circ 
213, 30 pp., illus. 



1924. COMMERCIAL FRUIT AND VEGETABLE PRODUCTS; A TEXTBOOK FOR 

STUDENT, INVESTIGATOR, AND MANUFACTURER. 530 pp., iUus. 

New York. 

(7) and Christie, A. W. 

1921. SOME FACTORS OF DEHYDRATER EFFICIENCY. Calif. Agr. Expt Sta 

Bull. 337, pp. [277]-298, illus. 

(8) and Christie, A. W. 

1921. DEHYDRATION OF FRUITS (A PROGRESS REPORT). Calif. Agr. Expt Sta 

Bull. 330, pp. [49]-77, illus. 

(9) Culpepper, C. W., and Caldwell, J. S. 

1927. THE RELATION OF ATMOSPHERIC HUMIDITY TO DETERIORATION OF 

EVAPORATED APPLES IN STORAGE. Jour. Agr. Research 35- 
889-906. 

(10) and Moon, H. H. 

1937. FACTORS AFFECTING THE RATE OF DRYING OF KIEFFER PE ^RS. 

U. S. Dept. Agr. Tech. Bull. 592, 31 pp., illus. 

(11) Gore, H. C, and Mangels, C. E. 

1921. THE RELATION OF MOISTURE CONTENT TO THE DETERIORATION OF 
RAW-DRIED VEGETABLES UPON COMMON STORAGE. Jour. InduS. 

and Engin. Chem. 13:523-524. 

(12) LuTz, J. M., Culpepper, C. W., and Moon, H. H. 

1934. THE RELATIONSHIP OF RIPENING TEMPERATURES TO THE RATE OF 
SOFTENING, TEXTURE, AND FLAVOR OF KIEFFER PEARS. Amer. 

Soc. Hort. Sci. Proc. (1933) 30:229-232, illus. 

(13) Culpepper, C. W., Moon, H. H., and Myers, A. T. 

1933. OBTAINING OPTIMUM DESSERT AND CANNING QUALITIES FROM THE 

KIEFFER pear; SOME INFLUENTIAL FACTORS. Canning Age, 
14:404-406, 414, 428. 

(14) Magness, J. R. 

1920. INVESTIGATIONS IN THE RIPENING AND STORAGE OF BARTLETT 

PEARS. Jour. Agr. Research 19:473-500, illus. 

(15) — and Taylor, G. F. 

1925. an IMPROVED type of pressure TESTER FOR THE DETERMINATION 

OP FRUIT MATURITY. U. S. Dept. Agr. Circ. 350, 8 pp., illus. 

(16) Malcolm, O. (Powell). 

[1919]. SUCCESSFUL CANNING AND PRESERVING, A PRACTICAL HANDBOOK 

FOR SCHOOLS, CLUBS, AND HOME USE. Ed. 3, rev., 405 pp., 
illus. Philadelphia and London. 

(17) Mann, C. F. A. 

1936. MECHANICAL PREPARATION OF PEARS ON THE COAST. Canning Age 

17:163-164, 184, illus. 

(18) Moon, H. H., and Culpepper, C. W. 

1934. FACTORS AFFECTING THE QUALITY OF PRESERVES MADE FROM 

KIEFFER PEARS. Fruit Products Jour. 14(1): 12-16. 

(19) Nichols, P. F., and Christie, A. W. 

1930. DEHYDRATION OP GRAPES. Calif. Agr. Expt. Sta. Bull. 500, 31 pp., 
illus. 

(20) Powers, R., Gross, C. R., and Noel, W. A. 

1925. COMMERCIAL DEHYDRATION OF FRUITS AND VEGETABLES. U. S. 

Dept. Agr. Bull. 1335, 40 pp., illus. 

(21) Thelen, R. 

1923. KILN DRYING HANDBOOK. U. S. Dept. Agr. BuU. 1136, 64 pp., 
illus. 

(22) TiEMANN, H. D. 

1917. THE THEORY OF DRYING AND ITS APPLICATION TO THE NEW HUMIDITY- 
REGULATED AND RECIRCULATING DRY KILN. U. S. Dept. Agr. 

Bull. 509, 28 pp., illus. 

(23) WiEGAND, E. H. 

1923. RECIRCULATION DRIERS. Oreg. Agr. Expt. sta. Circ. 40, 11 pp., 
illus. (Revised, 1929.) 

U. S. GOVERNMENT PRINTING OFFICE: 1937 



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